Many imagers and other integrated circuits (ICs) include conductive structures formed over a substrate's surface. For example, the conductive structures could be gates of transistors or electrodes of capacitors. Such structures are often formed of doped semiconductor material, such as doped polysilicon.
Typically, an imager IC includes a focal plane array of pixel cells, each one of the cells including a photoconversion device such as, e.g., a photogate, photoconductor, or a photodiode. In a CMOS imager, each pixel cell also includes readout circuitry, typically including a source follower output transistor. The photoconversion device converts photons to free electrons, which are typically transferred to a floating diffusion region connected to the gate of the source follower output transistor. A charge transfer device (e.g., transistor) can be included for transferring charge from the photoconversion device to the floating diffusion region. In addition, such imager cells typically have a transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transfer. The output of the source follower transistor is a voltage output on a column line when a row select transistor for the row containing the pixel is activated.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc. The disclosures of the foregoing patents are hereby incorporated by reference in their entirety.
In a CMOS imager in which each pixel cell includes a photodiode, when incident light strikes the surface of the photodiode, electron/hole pairs are generated in the p-n junction of the photodiode. The generated electrons are initially collected in the n-type region of the photodiode. The photogenerated charge moves from the initial charge accumulation region to the floating diffusion region or it may be transferred to the floating diffusion region via a transfer transistor. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower transistor (described above).
Some conventional imagers employ polysilicon in the gate stacks of the pixel transistors. Transistors with polysilicon gates, but without a metallic material (e.g., metal silicide) on the polysilicon, can have high gate resistivity problems. High gate resistivity can decrease operational speed. Accordingly, some imagers have attempted to alleviate the problem by using tungsten silicides (WSix) on the tops of polysilicon gates. However, complex process steps are required to form WSix polysilicon gates and it becomes more difficult to define both n-channel and p-channel metal-oxide-semiconductor field effect transistors (MOSFETs) with WSix gates. Other problems, such as cross dopant contamination between NMOSFETs and PMOSFETs, are more likely to occur during the production of WSix polysilicon gates.
Moreover, a blanket deposition of a silicide forming material can be detrimental to a photoconversion device. For example, high dark current can occur due to tungsten contamination of the photoconversion device area during the gate etch process when tungsten attacks the surface of the photoconversion device. It would be advantageous to have improved conductive structures with silicides over doped silicon material, and also to have improved techniques for producing such structures.